Tuesday 23 February 2010 16.49 EST
First published on Tuesday 23 February 2010 16.49 EST

Voters are worse off today than at the last general election in 2005, the first time in more than 50 years that wealth has declined over the lifetime of a full parliament, the Conservatives will claim tonight as they accuse Labour of presiding over a broken economy.

George Osborne, the shadow chancellor, will cite research showing that gross domestic product per person has declined by £281 as he revives one of Ronald Reagan's most famous sayings to lambast Labour. Reagan captured the mood during the 1980 US presidential campaign when he turned to the camera and asked voters in his final televised debate with Jimmy Carter: "Are you better off now than you were four years ago?"

Osborne will echo the hero of the global conservative right when he delivers the Mais lecture and says that the current parliament will buck a 50-year trend by presiding over a decline in national wealth. GDP per capita was £20,883 in the third quarter of 2005, a figure that has declined by £281 to £20,602 in the second quarter of this year. The figures are at 2005 prices adjusted for inflation.

In the 2001-2005 parliament GDP per head increased by £1,502. In the last parliament controlled by the Tories – 1992 to 1997 – GDP per head increased by £2,155. In only one parliament since 1955 has wealth declined. But that was the short parliament, which lasted between February and October 1974, when GDP per head declined by £14. Even in the turbulent years 1970 to 1974, wealth per head increased by £811.

Speaking before tonight's lecture, Osborne said: "When people ask the famous question, 'Are you better off than you were five years ago?' Gordon Brown is the first prime minister in modern British history who has to answer 'No'. Labour's 2005 manifesto promised increased prosperity. That is the biggest broken promise of all. Even through the dark days of the 1970s and the recessions of the early 1980s and 1990s, the growth of GDP per capita was sustained in every full parliament. This shows that the debt-fuelled model of growth that Gordon Brown pursued for the past decade is fundamentally broken. Gordon Brown's debt is the single biggest threat to our economic future. We need a new economic model built on saving and investment."

The Tories regard tonight's lecture to the Cass Business School at City University, London as a key moment for Osborne, who has faced criticism in the City for failing to demonstrate the gravitas expected of an aspiring chancellor.

The Mais lecture, which dates back to 1978, is one of the most prestigious events in the economic calendar. Geoffrey Howe, the last Tory shadow chancellor to take charge of the Treasury, in 1979, used the lecture in 1981 to deliver a landmark defence of Thatcherism under the heading: The Fight Against Inflation. Michael Heseltine was the last Conservative to deliver the lecture, in 1996.

Osborne's speech comes amid growing alarm at senior levels of the party as a series of polls show a narrowing of the Tory lead. A Guardian/ICM poll yesterday showed that the Tory lead has been cut to seven points, suggesting that David Cameron may lead the largest party in a hung parliament.

Cameron will tonight seek to calm Tory nerves when he addresses the backbench 1922 committee in advance of the party's spring forum in Brighton at the weekend, the last setpiece event before the general election. "David will want to gee up the troops," one frontbencher said. "We shouldn't get too concerned about movement in the polls which is not massive. But there is nervousness, though this is mainly on the basis that our great campaign launch in early January went off at half-cock."